Age-related changes in mental function are associated with, and are presumably due to, morphological and molecular alterations in the central and peripheral nervous systems. Most neurobiologists agree that some of these changes involve synapses, but relationships of aging to synaptic alteration are poorly understood. The neuromuscular junction (NMJ) is an ideal system to examine age-associated structural and functional alterations in synapses, and to elucidate their molecular bases. Exciting preliminary results show that aging causes major morphological and molecular changes to the pre- and post-synaptic components of the NMJ. Presynaptically, there are fewer axons, more varicose axons and more axonal sprouting. The postsynaptic sites are often fragmented and partially innervated. Concomitant with these morphological changes, three synaptic adhesion molecules, laminin alpha4, laminin beta2 and agrin, are significantly decreased in aged NMJs. Supporting a role for laminin alpha4 in aging, its deletion causes premature synaptic aging in mice. Surprisingly, old age and the absence of laminin alpha4 do not alter the morphology of extraocular NMJs, a muscle also spared in the neuromuscular disease, amyotrophic lateral sclerosis (ALS). These novel findings suggest that laminin alpha4, laminin aeta2, agrin and their cognate receptors may play a critical role in synaptic aging and neurological disorders. To begin uncovering the role of these molecules in aging synapses, I will first assess when and how these molecules decrease using immunohistochemistry and in situ hybridization. I will also seek age-related alternations in their receptors, such as MuSK, Integrins and P/Q-type calcium channels. To test for a causal role for laminin beta2 and agrin in aging, I will use conditional and heterozygous mice to assess if their absence or substantial decrease causes defects in young adult NMJs. Finally, to probe the relationship of aging to ALS, I will assess the expression of synaptic adhesion molecules in a mouse model of ALS. Together, these studies may allow me to identify molecules that can attenuate or even reverse age-induced deleterious effects on synapses. Consequently, these experiments could provide tools to ameliorate a number of synaptic defects underlying cognitive and spinalmuscular diseases.